1. Field of the Invention
This invention relates to a micro lens array in which a plural of a convexly protruded micro lenses are closely arranged in two dimensions and a plural of convex sub-lenses having a smaller curvature radius than that of the micro lenses are protrusively formed along the surface of each micro lens. The present invention also relates to a method of manufacturing a replication mold for the micro lens array.
2. Description of the Related Art
The micro lens array configured by arranging a plural of convexly protruded micro lenses closely in two dimensions is used for a liquid crystal display, an optical coupler element, an image input device, and the like.
Making use of optical anisotropy, orientation, fluidity and the like of a liquid crystal molecule, a liquid crystal display creates images using a liquid crystal cell on which an optical shutter is arranged the shutter being capable of changing an optical transparency or reflectivity thereof on application of voltage to the cell. The liquid crystal display is categorized into two types: a direct-view-type display in which images created by liquid crystal cells are directly observed; and a projection-type display in which images created on a screen by being projected from the front or the back are observed.
The above-mentioned direct-view-type display is somewhat disadvantageous in that image quality changes with observation angles. In other words, while the display has the highest brightness when seen from the normal direction, the brightness becomes lower when the observation angle deviates from the normal direction. At worst, the display is unobservable when seen from a certain view angle or above. That is, this type of display has a disadvantage in that an appropriate angle of view is narrow.
In order to eliminate this disadvantage, there has been proposed a method of increasing an angle of view by combining a liquid crystal display and a micro lens array, in other words, a method of disposing in front of a liquid crystal cell a micro lens array configured by arranging a plural of micro lenses closely in two dimensions.
Also, there has been proposed a use of the micro lens array for a screen so as to expand the angle of view in the projection-type display in which images are created on a screen by being projected from the back.
An example of this type of micro lens array is a micro lens array configured by disposing a plural of micro lenses having a convex shape in a hexagonal close-packed (honeycomb) arrangement as disclosed in Japanese Patent Application Publication No. 2001-305315 (for example on page 7, FIG. 7).
In addition, as a method of manufacturing a replication mold for a micro lens array configured by arranging a plural of micro lenses in two dimensions, the method utilizing an etching or sandblasting technique, has been known, as disclosed in Japanese Patent Application Publication No. H10-62604 (pages 3–6, FIG. 1).
Furthermore, there have been disclosed a lenticular lens sheet having fine asperity on the surface of a lenticular lens device (see for example, Japanese Patent Publication No. 3,212,359 (pages 3–4, FIG. 1), a micro lens array having a compound spherical surface on which two or more types of fine convex-concave shapes are formed, and a method of manufacturing the same (see for example Japanese Patent Application Publication No. H07-63,904 (pages 3–4, FIG. 1)).
FIGS. 1A, 1B and 1C are a front view, a perspective view, and a bottom view of a related-art micro lens array element, respectively. FIGS. 2A to 2C are a process chart showing a related-art method of manufacturing an optical substrate (a replication mold for a micro lens array).
The related-art micro lens array element 100 shown in FIGS. 1A to 1C is disclosed by the above-mentioned Japanese Patent Application Publication No. 2001-305315. The micro lens array element 100 is explained briefly, referring to FIGS. 1A to 1C.
The related-art transparent micro lens array element 100 includes a lens substrate 101 manufactured using a transparent substrate such as a transparent resin substrate, a resin sheet, or the like, and a plural of micro lenses 102a projected in a shape of a near hemisphere, the micro lenses 102a being integrally arranged in two dimensions on an upper surface 101a of the lens substrate 101 as shown in FIGS. 1A to 1C.
When the plural of micro lens 102a constituting the micro lens array 102 are seen from a bottom face 101b of the lens substrate 101, they are arranged in a hexagonal close-packed (honeycomb) arrangement and thereby placed at the highest density.
On the bottom face 101b of the lens substrate 101, the micro lens array 102 has a light incidence portion (or light exiting portion) 103 in a position which is in agreement with an optical axis K of each micro lens 102a and a light shielding portion 104 which prevents light from passing therethrough in a position other than each light incidence portion (or light exiting portion) 103. Each light shielding portion 104 has a diffusion reflection film (or an anti-reflection film) 105 thereon. Japanese Patent Application Publication No. 2001-305315 states that images of high luminosity and high contrast in a large angle of view can be realized with this construction when the micro lens array element 100 is applied to a liquid crystal display, a rear projector equipment or the like.
Next, the method of manufacturing an optical substrate (a replication mold for a micro lens array) disclosed in Japanese Patent Application Publication No. H10-62604 will be explained briefly, referring to FIGS. 2A to 2C.
A method of manufacturing the optical substrate is actualized in two types depending on whether an etching or sandblasting technique is employed. Hereinafter, these methods are explained in this order.
Firstly, where etching is employed, a photoresist 202 is coated on an upper surface 201a of a glass substrate 201 as shown in FIG. 2A, and then made into a mask by photolithography. The mask has an opening for an alignment mark portion AM that is to form an alignment mark and an opening for a micro lens portion R that is to form a micro lens.
Next, as shown in FIG. 2B, using the photoresist 202 on the glass substrate 201 as the mask, the glass substrate 201 is etched so as to form the lens portion R and the alignment mark portion AM therein by wet etching with an HF etchant, or dry etching with carbon tetrachloride (CCl4) gas.
While the photoresist 202 is removed in the lens portion R during the etching, the photoresist 202 remains in the alignment mark portion AM since the photoresist 202 is patterned so that the photoresist 202 is not removed in the alignment mark portion AM by etching, as shown in FIG. 2C. After the above procedures are carried out, the optical substrate (namely, a replication mold for a micro lens array) 201 that serves as a mold for manufacturing a micro lens array (not shown) is completed.
Although not illustrated, by filling a transparent resin having a high refractive index into the lens portion R and the alignment mark portion AM shown in FIG. 2C, a micro lens array is obtained.
On the other hand, when employing the sandblasting, a photosensitive dry film photoresist having a resistance to sandblasting is used as a photoresist 202. Then, an opening in this dry film is formed in a position in which the lens portion R should be formed and a position in which the alignment mark part AM should be formed by photolithography. Next, in the sandblasting, the glass substrate 201 is ground physically, thereby forming the lens portion R and the alignment mark portion AM. Then, the sandblasted surface of the glass substrate 201 in which the lens portion R and the alignment mark portion AM have been formed is slightly etched by wet etching with the HF etchant or dry etching with the CCl4 gas, in order to make the surface flat and smooth. With the above procedures, the optical substrate (a replication mold for a micro lens array) 201 that serves as a mold for manufacturing a micro lens array (not shown) is completed.
Although the related-art micro lens array element 100 shown in FIGS. 1A to 1C is able to increase an angle of view when applied to a liquid crystal display, a rear projector or the like, a further increase in the angle of view has been desired.
In addition, when the etching is employed in the related-art method of manufacturing the replication mold as shown in FIGS. 2A to 2C, only a small change in a chemical composition or crystal structure of the substrate 201 may lead to a variation in etching characteristic. When this happens, a desired shape of the lens portion R and the alignment mark portion AM is no longer obtained. Moreover, when the optical substrate is not sufficiently cleaned immediately after the substrate is etched into a desired shape, it is difficult to maintain the desired shape of a replication mold having a fine lens structure because the remaining etchant may erode the substrate.
When the sandblasting is employed in the related-art method of manufacturing the replication mold, the photosensitive dry film photoresist 202 remains on the substrate after the sandblasting because the dry film photoresist 202 has a resistance to sandblasting, thereby leaving a distance between the adjacent two lens portions R. The distance cannot be eliminated even when etching is carried out in order to smoothen the sandblasted surface of the lens portion R and the alignment mark portion AM, thereby making it difficult to dispose a plural of lens portions R at the highest density.
A micro lens array that enables a further increase in the angle of view rather than the conventional micro lens array and a method of manufacturing a replication mold for such a micro lens array have been desired.